I'd really like to stop blowing out the sky and/or underexposing the ground in my pictures. The traditional solution would be to use graduated ND filters, but taking multiple exposures and applying HDR postprocessing would also work (and some point-and-shoot cameras can even do that in-camera).

When should I use graduated ND filters, and when would HDR be a better bet? Or is it just a matter of personal preference?

You don't need to use HDR processing. You can create multiple exposed images, and composite them instead. This takes work in PP, but results in a closer true to life image, without having to make too make a correct tonemap.
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AlanJul 20 '10 at 17:49

+1 for not wanting the "HDR Look" .. I'm so sick of looking at highly detailed urinals and shower stalls on flickr!
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Tim Post♦Jul 20 '10 at 15:29

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Couldn't agree more with Tim! Technically speaking, the "HDR Look" is just improper tone mapping anyway, and is not actually the correct look for an HDR image. HDR simply gives you a greater bit depth with a broader viable range of contrast, which is exactly what we photographers need. ;)
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jrista♦Jul 20 '10 at 16:43

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Reid: Probably want to throw in cost as well. Good ND filters aren't cheap. And if you have lenses with different filter sizes, you'll need ND's for each.
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AlanJul 20 '10 at 17:47

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@jrista - I'm not saying its never appropriate to abuse tone mapping, I've seen some really creative uses of it particularly in industrial decay. I just wonder why everyone's first HDR is a toilet. Its like toilets are the "Hello, World!" of HDR.
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Tim Post♦Jul 21 '10 at 13:47

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Additional Disadvantages of HDR: (1) You camera needs to support bracketing and preferably bracketing with many frames. (2) You can't do it for moving subjects - if something is moving in the shot, you need to edit it out later.
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DimitrisJul 22 '10 at 21:32

I'll just add that there are alternatives to HDR tone-mapping. Sometimes manually blending layers in photoshop works well (and gives you a more natural looking result, similar to using a grad filter except you have a lot more control over the transition).

Another good option is exposure fusion, as implemented in Tufuse and Tufuse Pro. This gives a more automated approach than manual blending, but without the stylized look that usually comes out of Photomatix. The only downside is that it doesn't compensate for ghosting caused by movement in-between exposures.

I think what you need to consider is the fact that using HDR won't just fix your blown skies - it's an effect in its own right and will increase the dynamic range of the whole image. If you want to shoot HDR anyway, then you'll be able to expose your skies correctly due to the increased dynamic range of the image.

If you don't want a HDR affect on your photo use a filter (real or in software, although personally I'd always use a real filter and get it right in-camera) to bring the sky into the dynamic range of the rest of the image.

HDR isn't something that interests me too much at the moment, so I'd always use a filter. Essentially what I'm saying is that you should use a filter unless you happen to be shooting a HDR image anyway, in which case it probably isn't necessary.

There are some good answers already, so I'll just add some notes. HDR is a powerful tool that can be used to produce images with a much broader range of contrast than is normally possible with a camera. It should be noted, however, that if you really do "blow out" your highlights by overexposing beyond the upper range of your sensor, or conversely "block out" your shades by underexposing beyond the lower range of your sensor...no amount of HDR will be able to correct that. Once you go far enough beyond the absolute limits of your sensor (truly blow out), HDR can't help you. You will still need to be careful that, even if you ride the line and get right up to the edges of your sensors contrast range, you don't go beyond. HDR can then serve you very well.

Isn't the whole point of HDR in correcting blown out hightlights by merging the image with another exposure that has captures highligt details?
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cheJul 20 '10 at 20:36

@che: The point of HDR is to gain increased viable contrast range. If you expose far enough beyond the physical limits of your sensor (you might be able to go a little beyond, but not much), no amount of HDR processing can correct that. I have several very important photos that I accidentally overexposed...not by much, but a little beyond the abilities of my sensor. I've used numerous programs to try and "fix" them, but the most over exposed parts are always visible...there is either some degree of posterization, or they don't look quite right, or something. Now...
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jrista♦Jul 20 '10 at 21:55

...when shooting in RAW, you can "overexpose" a photo by quite a bit at times... 2/3 of a stop at least, sometimes up to a stop or a little more, without actually exposing past the physical limits of your sensor. Thats one of the benefits of using RAW. However if you want to get the best results from HDR processing, you should make sure that those "overexposed" highlights (or shadows) are recoverable, and are not physically unrecoverable (blocked or blown.) If you have only gone a little past the physical limits, HDR can still help you recover...but you may be stuck with some artifacts.
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jrista♦Jul 20 '10 at 21:57

I personally tend to use a gradient transition in post to mimic graduated neutral density filters. Lightroom does this as one of its handy tools, but you can also use it in Photoshop. Just create a layer with a gradient between white and black with a transition abruptness you select across the sky/ground transition.

That way, you don't have to stress the HDR, nor lug around a huge number of filters.